Existence of a sodium‐induced calcium release mechanism of frog skeletal muscle fibres.

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Abstract
The electrical and the mechanical activity of isolated frog muscle fibers were simultaneously recorded in a physiological solution which allows the development of a large tubular Na current. Under such experimental conditions, fibers develop long-lasting action potentials and strong mechanical responses. In voltage-clamp experiments a slow inward current is revealed for depolarizations higher than +20 mV from the resting potential. This current increases until +40 to +50 mV and then decreases to reverse near +90 mV. The amplitude of the mechanical response increases with the potential to reach an optimum value between +40 and +50 mV and then decreases to stabilize when the depolarization is near +90 mV. In the presence of picrotoxin the slow inward current is reversibly inhibited and the tension-depolarization curve has an S-shape as found in normal physiological conditions. The dependence of a part of the contraction upon the slow inward current is reinforced by the fact that in a 50% Na solution the amplitude of the current and that of the contraction are reduced in the same proportion. Detubulated fibers failed to generate such a Na inward current. When Na ions are replaced by Li ions a slow inward Li current develops, but it does not induce a mechanical response. Tetracaine reversibly inhibits the current-dependent component of the contraction without affecting the potential-dependent one. The contraction recorded in the present experimental conditions is the sum of 2 components: one is potential-dependent and the other depends on a Na-induced Ca release mechanism.